1. Field of the Invention
The present invention relates generally to a distributed amplifier having a variable terminal resistance, and in particular to a distributed amplifier having a variable terminal resistance, in which a load resistance of a signal amplification unit thereof may be adjusted to achieve the optimal resistance matching so as to obtain a good gain flatness.
2. The Prior Arts
FIG. 1 is a circuit diagram of a conventional distributed amplifier. The distributed amplifier comprises a signal amplification unit A, a first biasing circuit B, a second biasing circuit C, a terminal resistance D, a radio frequency (RF) signal input terminal E and a RF signal output terminal F. The signal amplification unit A is used to amplify a RF input signal fed at the RF signal input terminal E. The first biasing circuit B is used to provide a direct current (DC) bias at a base of a transistor A1. The second biasing circuit C is used to provide a DC bias at a collector of the transistor A1. The terminal resistance D is used to provide a resistance matching with an output resistance of the distributed amplifier. As also mentioned above, the RF signal input terminal E is used for a RF signal input. The RF output terminal F is used for a RF signal output.
In the above distributed amplifier, the signal amplification unit A has a plurality of transistors A1, a first external inductor A2, a second external inductor A3, a third external inductor A4 and a fourth external inductor A5. In the signal amplification unit A, the first external inductor A2 is electrically connected to a base of the transistor A1. The base of the transistor A1 is electrically connected to a base of a next transistor A1 via a coupling inductor L. In this manner, the next transistor A1 is further electrically connected to its next transistor A1 at their bases via another coupling inductor L, and the final transistor A1 is electrically connected to the second external inductor A3 at the base. Meanwhile, the third external inductor A4 is electrically connected to the collector of the transistor A1. The transistor A1 is electrically connected to its next transistor A1 at their collectors via another coupling inductor L. In this manner, the next transistor A1 is electrically connected to the collector of its next transistor A1 at its collector via another coupling inductor L and the final transistor A1 is electrically connected to the fourth inductor A5 at its collector. The first bias circuit B is electrically connected to the second external inductor A3. The terminal resistance D is electrically connected to the third external inductor A4. The second bias circuit C is electrically connected to the external resistance D. The RF signal input terminal E is electrically connected to the first external inductor A2. The RF signal output terminal F is connected to the fourth external inductor A5 via the fourth inductor A4.
The terminal resistance of the distributed amplifier has a constant value, which does not always match with the output resistance of the signal amplification unit since the output resistance will vary as the DC bias voltage of the signal amplification unit is changed so as to achieve a different gain. This can adversely affect the gain flatness provided by the distributed amplifier.
In light of the demerits encountered in the prior art, the inventor of the present invention sets forth a distributed amplifier in which a variable terminal resistance is provided for resistance matching to improve the fain flatness issue.